Seoul: New US Secretary of State John Kerry and his South Korean counterpart have agreed to make sure North Korea is punished if it carries out its threat to conduct a nuclear test.

Under a UN Security Council resolution last month condemning a North Korean long-range rocket launch that the UN and others call a disguised test of banned missile technology, Pyongyang is subject to new sanctions if it detonates its third nuclear device since 2006.

North Korea announced last month that it will conduct a nuclear test to protest the toughened sanctions over its December launch, which delivered a satellite into orbit.

During a phone conversation between Kerry and South Korean Foreign Minister Kim Sung-hwan, "there was agreement that… if the DPRK continues its provocative behavior and takes further steps, that there must be further consequences," State Department spokeswoman Victoria Nuland said on Monday.

DPRK is the acronym for North Korea's official name, the Democratic People's Republic of Korea.

A North Korean nuclear test "seems to be imminent," South Korea's UN Ambassador Kim Sook on Monday said at a news conference at UN headquarters in New York.

He said there are "very busy activities" taking place at North Korea's nuclear test site "and everybody's watching."

The ambassador said he expects the Security Council to respond with "firm and strong measures" in the event of a nuclear test.

North Korea has denounced sanctions over its rocket launches, saying it has the sovereign right to use rockets to send satellites into orbit under a space development programme.

Pyongyang's two previous nuclear tests, in 2006 and 2009, both occurred after it received international criticism for similar rocket launches. As it issued its most recent punishment, the Security Council ordered North Korea to refrain from a nuclear test or face "significant action."

North Korea's state media said on Sunday that at a high-level Workers' Party meeting, leader Kim Jong Un issued "important" guidelines meant to bolster the army and protect national sovereignty.

North Korea didn't elaborate, but Kim's guidelines likely refer to a nuclear test and suggest that Pyongyang appears to have completed formal procedural steps and is preparing to conduct a nuclear test soon, according to South Korean analyst Hong Hyun-ik.